i imported an FBX file and want to add bones
When importing models from other 3D computer graphics programs the model size is one of the main problems you will run into. The reason for this is that 3D computer graphics is not (necessarily) CAD. You don't have unified measurement units. Blender has a different internal measurement system than 3ds max, which uses a different one than Maya, and so on.

You need to resize your model to a reasonable size. The default grid floor is a good orientation point. 



You also want to change Blender's internal measurement units from "None" to "Metric" or "Imperial" (Metric works great for me). 

Go to Properties Window, Select "Scene" panel, scroll down to "Units" and change the settings to your liking. 



This comes also in handy when exporting models to game engines.

Also be aware that the model's mesh isn't already triangulated. You simply won't have very much fun rigging it.